(a) Field of the Invention
The present invention relates to a liquid immersion plan apochromat type objective lens system for microscopes which is designed for a magnification of .times.100, a numerical aperture (N.A.) of approximately 1.4 and a field number attaining to 30.
(b) Description of the Prior Art
It is one of the difficult problems to design a plan apochromat type high magnification objective lens system for microscopes having a very large N.A. of approximately 1.4. The reason for such difficulty lies in the fact that it is required to favorably correct chromatic and spherical aberrations by adequately distributing powers among lens components and selecting suitable materials therefor, and at the same time, to minimize Petzval's sum. However, there are conflicting tendencies in these corrections. That is to say, an attempt to minimize Petzval's sum will aggravate chromatic aberration, etc., whereas an attempt to correct chromatic and spherical aberrations, in contrast, will increse Petzval's sum, thereby posing the afore-mentioned difficulty. It is a historically known fact that chromatic aberration can be corrected by selecting glass materials having different dispersing powers for positive and negative lens components. In case of an apochromat, chromatic aberration is further corrected by utilizing difference in dispersion by using extraordinarily dispersive glass such as fluorite. On the other hand, it is also known well that Petzval's sum can be minimized by adequately arranging wide airspace, thick lens component and negative lens component in a lens system. In an objective lens system of this type, arrangement of negative lens components are important for correcting aberrations and minimizing Petzval's sum.